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Author | Topic: Exploring the Grand Canyon, from the bottom up. | ||||||||||||||||||||
jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
Sure would like for one of our
Edited by jar, : No reason given. Aslan is not a Tame Lion
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jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
Just a few questions if you please.
You say "a series of basalt and basaltic andesite flows and sandstone interbeds." For the benefit of this OLD mind, does that mean the Cardenas layer is actually a whole series of layers? What does "Andesite flows" mean? Just for those following along we are approaching 200 posts and here is where we are.
Aslan is not a Tame Lion
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jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
Okay, got that it is a different type of lava.
Need some more help. Can one event produce both types of flows? Edited by jar, : appalin spallin Aslan is not a Tame Lion
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jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
It seems that the Cardenas Lava flows are between 685 and 985 feet thick.
The lower Dox formation we covered earlier is mostly shale (mud) and sandstone with indications of being made during a series growing and receding seas. It has areas that show ripple marks and also area of mud cracks. While the Cardenas Lava flows are between 785 and 985 feet thick, the Dox formation is up to 3000 plus feet thick. As we look at each of these layer we seem to find signs that each layer really was at one time at the surface; for example we found ripple marks, rain drop impressions and mud cracks in parts of the Hakatai Shale (about 400 to 100 feet thick), then the Shinumo Quartzite layers (1000 to 1300 feet thick) then the Dox formation that is a totally different set of compositions and, as stated about, ranging up to 3000 feet thick. Is that a reasonable summary so far? Aslan is not a Tame Lion
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jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
The Nankoweap Formation is a 370-foot (113-meter) thick cryptic unit that comprises sandstone and lesser amounts of siltstone, shale (rare black shales), and mudstone (Elston and Scott, 1976). This formation has been informally subdivided into an upper and lower member. This is based both on variations in grain size (the lower member is finer than the upper member), but more importantly by an angular unconformity that separates the two members and signifies tectonic activity during Nankoweap time (Elston and Scott, 1976). From this source. Above the Cardenas formation we find the Nankoweap. Once again we see a change in the types of rock From what little I have learned so far, several thing jump out at me. First, it mentions that the lower part of the formation has a finer grain composition than the upper. That seems to indicate that the lower layers were deposited in a less active environment than the upper. I understood that when material is transported by both water and air, the larger particles get deposited first and the finer material is held in suspension longer. If the Nankoweap had been a single event I would have expected the larger coarser material to be lower with the finer sediment towards the top. Is that reasonable? Second, they mention an angular unconformity. (see Young earth explanations for Angular Unconformities.)Now please correct me if I am wrong. If I remember correctly from earlier parts of this thread, sandstone, siltstone, shale and mudstone each form under different conditions and have different compositions. Can you or another of our geologists give me a short description of each of those just as a reminder? Pictures would also help. Aslan is not a Tame Lion
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jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
Can we hold this for just a short while. See Message 199.
The reason is that the Bass formation will return in just a little while after we cover the last few groups and at that time it will become very pertinent. Edited by jar, : fix link Aslan is not a Tame Lion
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jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
Are there any geologists that can help me with some issues on the Nankoweap formation?
See Message 205. I'd really like to at least work through the Super Group before we need to close this thread and even after we cover these last few formations, I have some really big questions to ask about what happened. Aslan is not a Tame Lion |
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jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
Thanks. That helped a lot in differentiating them.
But is there any chance I could get you to elaborate a little on how they are made? Aslan is not a Tame Lion
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jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
I guess I'm just old, but why does one formation end up sandstone, another shale or mudstone?
Aslan is not a Tame Lion
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jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
Okay, let me try describing it and see how close I come.
If we looked at the rocks and asked about how the materials that got there might have been transported, we could relate material laid down to how active the transport method was by size of particulate. From most to least active we would see:
Is that correct? Aslan is not a Tame Lion
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jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
The next group up is the Chuar Group. From what I gather it is yet another major change from the lower Nankoweap and made of at least two major formation.
What can you tell me about the Chuar Group other than that it is almost a mile think? Aslan is not a Tame Lion
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jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
Welcome to the thread and thanks for reminding us.
What we are trying to do in this thread is as the title suggest, slowly work our way up formation by formation from the current bottom of the Grand Canyon. So far we are still down in the basement and what we are trying to do is actually describe that hands on aspect of each layer.
The important thing to think about when contemplating sedimentary rocks is the process of formation. That is what we are looking for, with the understanding that dating should be avoided for now. Try and think of this thread as a view as it might have been seen several hundred years ago, long before things like radiocarbon dating were known. Aslan is not a Tame Lion
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jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
Great, please hang around because once we cover these last few formations in the Super group we get to the exciting questions of how they got tilted and where so much of them went?
Aslan is not a Tame Lion
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jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
"hand sample" means?
"very thin laminae which are parallel to bedding (bedding is very roughly analogous to the amount of sediment laid down during one sedimentation event--one event more or less equals one bed)." means? "So where as mudrocks are bedded sedimentary rocks, shales are bedded and laminated sedimentary rocks. " means? Now I do remember what wedded and bedded meant, least wise I think I do, but now you tossing in lots of new stuff. Are you saying that if I found a layer of mudstone that I would be looking at one event?
For most of the discussion we're likely to have here, we don't really need to worry about the differences between mudrocks and shale and we can think of them as roughly equivalent (although in reality they form under slightly different conditions and thus are different rocks). Say what? Remember old and not to bright? What different conditions? Aslan is not a Tame Lion
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jar Member (Idle past 422 days) Posts: 34026 From: Texas!! Joined: |
Actually both useful and important.
One of the claims often made is that folk are just making assumptions when they talk about how some rock was made. But you say:
There are usually a lot of clues preserved in any given package of sediment telling us how they came to be, but it isn't always trivial to tease the answer out. For the benefit of those of us who are stumbling in the dark, what are examples of such clues and can you link to pictures of samples we could see that would help us understand such differences? AbE: Just for my benefit, look at the following pictures. I know I'm asking alot but if I came across something like these, what should they tell me?
Image 1 Image 2 Image 3 Image 4 Edited by jar, : add images and questions. Aslan is not a Tame Lion
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